1. Field of the Invention
The present invention relates to a laser scanning apparatus suitable for an image forming apparatus such as a printer and a copying machine with an electrophotographic system respectively, and in particular, to the emission control of a laser beam at the time of starting the laser scanning apparatus.
2. Related Background Art
Conventionally, in a laser beam printer which drives a rotary polygon mirror on the basis of a horizontal synchronizing signal (hereinafter, this is called a BD signal), for example, a method of performing configuration and control as disclosed in Japanese Patent Application Laid-Open No. 08-183198 is known.
That is, the rotation control of the rotary polygon mirror is performed by measuring a cycle of a horizontal synchronizing signal between adjacent BD signals, and determining the emission timing of a laser beam for acquiring a next BD signal every horizontal synchronization from the BD cycle measured.
Another method is also known, the method where this timing control is performed during start to the steady rotation of a rotary polygon mirror as well as the steady rotation so as to prevent the degradation of a photosensitive drum by not irradiating a photosensitive drum surface with a laser beam as much as possible, and to prolong the life time of a laser by shortening its lighting time.
However, it is necessary to change the lighting timing of the laser in real time in the above-described conventional example, even when a BD cycle greatly differ from the steady rotation not only under steady rotation but also under start.
Thus, this means that it is necessary to compute the optimal timing in real time every input of the BD signal so as not to emit the laser beam on a photosensitive drum surface. Hence, there is a problem that constitution and control become complicated.
The present invention is made under such circumstances. An object of the present invention is to provide a laser scanning apparatus that can prevent the degradation of a photosensitive drum and can prolong the life time of a laser by suppressing the unnecessary emission of a laser beam to the photosensitive drum with the simple configuration for timing control, an image forming apparatus using this laser scanning apparatus, and a method of starting the laser scanning apparatus.
The laser scanning apparatus of the present invention is characterized by comprising a rotary polygon mirror for scanning a laser beam, synchronizing signal generating means for detecting the laser beam scanned by the rotary polygon mirror and generating a synchronizing signal, measuring means for measuring a cycle of the synchronizing signal generated by the synchronizing signal generating means, laser emission control means for making a laser beam emitted in order to acquire a synchronizing signal for next scan, start speed measuring means for measuring increasing speed of rotation of the rotary polygon mirror from the cycle of the synchronizing signal measured by the measuring means in an early stage of starting rotation of the rotary polygon mirror, start time prediction means for predicting start time required for the rotary polygon mirror to attain steady rotation on the basis of information from the start speed measuring means, and control means for making a laser beam not emitted until the time predicted by the start time prediction means elapses from measurement completion by the start speed measuring means, making the laser beam emitted after the predicted time elapses, and operably controlling the laser emission control means when the cycle of the synchronizing signal measured by the measuring means reaches a predetermined cycle.
Other objects, configuration, and effects of the present invention will become apparent from the following detailed description taken in connection with the accompany drawings.